In the field of multi-cylinder engines having a plurality of cylinders, there has heretofore been known a technique, so-called “reduced-cylinder operation” in which a part of the cylinders is deactivated by stopping combustion therein.
During the reduced-cylinder operation, the number of activated cylinders is reduced, thereby possibly leading to a decrease in power output of the entire engine. Therefore, it is common practice to execute, during the reduced-cylinder operation, a control for increasing an actual intake air amount, i.e., an amount of air to be taken into each of the activated cylinders, so as to increase a power output of the activated cylinders.
In this case, however, the actual intake air amount is increased with a certain delay. Thus, if the control for increasing the actual intake air amount is started simultaneously with start of the reduced-cylinder operation, the actual intake air amount becomes deficient at start of the reduced-cylinder operation, thereby leading to the occurrence of a decrease in engine power output, i.e., torque shock, resulting in deteriorated ride comfort.
As one measure against this problem, it is conceivable to preliminarily increase the actual intake air amount before start of the reduced-cylinder operation. However, if the actual intake air amount is increased in a state before the start of the reduced-cylinder operation, i.e., in a state in which combustion is performed in all cylinders, an engine power output before the start of the reduced-cylinder operation becomes excessive, in this case.
In this regard, the following Patent Literature 1 discloses an apparatus configured to shift a throttle valve provided in an intake passage toward its fully-open side, before start of a reduced-cylinder operation, to increase an actual intake air amount to a value suitable for the reduced-cylinder operation, and controllably set an ignition timing in each cylinder to a retard side, to thereby increase the actual intake air amount while suppressing an increase in engine power output, before the start of the reduced-cylinder operation.